Hydraulically setting inorganic binders based on calcium and aluminum silicates have been known for a long time and are used in various compositions and with various properties; see, for example, main class C04 of the International Patent Classification.
Conventional hydraulic binders, e.g. Portland cement, hydraulic limes and calcium aluminate cements, harden to become brittle materials such as mortar and concrete, having high elastic moduli compared to their strengths. Furthermore, conventional hydraulic binders are characterized by a high content of calcium and a high ratio of calcium/silicon, and their strengths are determined by sufficient quantities of dissolved calcium in the pore solution. If the dissolved calcium is washed out of the binder matrix or is immobilized by carbonization, this may lead to a disintegration of the strength-forming mineral phases, i.e. calcium silicate hydrates. Consequently, the durability of binders based on calcium silicates is limited.
Furthermore, galvanic corrosion protection (GCP) has been used for many years for protecting steel in concrete and steel constructions, tubings etc. against corrosion, as described in AT A 1344/2004, EP 1,135,538, EP 668,373, and U.S. Pat. No. 4,506,485. The effect of GCP is based on the formation of a galvanic element between a sacrificial anode and the steel. If galvanic protection is used for protecting steel reinforcements in concrete, the concrete acts as an electrolyte. For protecting steel constructions, usually a gel-like flexible electrolyte is applied between steel and galvanic anode.
For the most part, the gel-like flexible electrolyte is present as an adhesive layer on a metal anode. As the anode material, usually zinc and its alloys, more rarely aluminum and its alloys are used. The anode is usually mounted on the surface of the construction element to be protected, in some cases it is introduced as “discrete galvanic anode” into concrete.
Disadvantages of the known sacrificial anodes in the protection of steel, especially reinforced concrete, against corrosion are that zinc passivates in contact with calcium ions, especially calcium hydroxide, and is inactivated after a short period. Known sacrificial anodes such as zinc applied to a concrete surface by means of a plasma spray method according to the Grillo KKS method (WO 2005/03061) are thus only effective at high humidity and high chloride contents. Once the system has dried out, the zinc passivates irreversibly. In order to avoid these disadvantages, so-called discrete anodes, as described in U.S. Pat. No. 6,572,760 (B2), have been developed. The problem of zinc passivation was solved by adding alkalis, usually alkali hydroxide, to the binder in which the zinc is embedded. Practice has shown that a pH of approximately 14 is required for sufficient activation of the zinc anodes. Thus, these discrete anodes may only be used on construction sites with substantial safety measures, as they are commonly used for highly alkaline corrosive construction materials. In addition, it has been shown that, in the medium term, alkalinity is reduced by galvanic reactions and that zinc passivates, especially in construction elements being exposed to dry-wet cycles. Furthermore, alkalis in concrete may have negative effects on the concrete's strength due to the alkali-silica reaction.
Against this background, it was the object of the invention to provide a hydraulic binder as well as a sacrificial anode made thereof with which the above disadvantages can be largely or entirely overcome.